Germfree rats and mice are being developed as experimental model systems for use in this Laboratory and by other qualified investigators. The projects include carcinomas of the breast, intestine, and prostate; myelogenous and lymphatic leukemias, and myelomas. The nature of senescence in germfree rats is being determined, the phenomenon of tumor metastasis is being examined, and therapeutic procedures (chemo- and immunotherapy) are being applied to the tumor model systems noted above. New strains of germfree rodents, as needed, are being developed.